<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>css z-index</title>
</head>
<body>

    <h1>这是一个标题</h1>

    <img src="../../img/bg.jpeg"
         style="position: absolute; left: 0px; top: 0px; z-index: -1">
</body>
</html>